There was a large hole in the pressure hull around one and a half meters to two meters, and now we know for sure that in result of that powerful blast for 75 or 80 percent of the crew died within 90 seconds since the submarine was at so-called periscope depth at the moment, which implies that all the crew were at battalion stations in the first two or three water-tight compartments.

To avoid this tell-tale an instrument called a periscope has been invented, which looks like a bottle on the end of a tube; this has lenses and mirrors that reflect into the interior of the submarine whatever shows above water.

The principle of the periscope is the same as that of the "busybody," familiar to householders, and which is placed on the sill of an upper window, so that a person inside the house may see who is at the front door.